This invention relates to a web alignment system. It relates more particularly to a system which maintains very accurate alignment of a web edge or center line to a fixed reference point.
There are numerous web alignment systems which monitor the lateral position of a travelling web with respect to a reference point and control steering rollers or the like to shift the web laterally as needed to maintain the web edge or the web center line at that reference point.
A variety of devices are used to sense web position. These include photo-electric devices which detect light reflected from the web, various types of pneumatic devices which sense the presence of web by an increase in the back pressure at a nozzle positioned opposite the web path and various types of mechanical fingers which sense when the web strays laterally from the desired reference position.
All of these prior devices must be repositioned whenever the width of the web being run changes to any appreciable extent.
Some prior systems do include manual trim adjustments for accommodating small changes in web width, e.g. one-tenth inch or less. However, any appreciable width change requires the manual repositioning of the web sensors. This operation can be time consuming, particularly when the web sensors are situated in congested and relatively inaccessible locations in the line.
Also, often it is desirable to align the web being edge-position controlled with another web, which may or may not be edge-position controlled, in order to accurately laminate or combine the two webs with minimum edge trim or variation. Prior sensors cannot easily be used to provide a position trimming signal to enable a second sensor to detect the difference between the edge position of a master web and a slaved or follower web.
Still further-conventional devices of the general type must be located in close proximity to the web being guided. In many applications due to machine interferences this is difficult or impossible to accomplish.
Also, in many applications, the web is center-guided in that the web center line is maintained at the fixed reference position. These applications require two web sensors positioned at opposite sides of the web to produce a difference signal that compensates for minor variations in the linearity of the web edges. The requirement of two sensors not only increases the initial cost of these prior center-guided systems, but also means that twice as much time is needed to reposition the sensors when different width web is run.
For the foregoing reasons, the prior arrangements for controlling the lateral position of a web have not been entirely satisfactory and have resulted in excessive down-time of associated web processing apparatus between web changes.